1. Field of the Invention
The present invention relates to an image forming apparatus for a printer and a copying machine, particularly to a color-image forming apparatus in which toner images of a plurality of colors are borne and conveyed.
2. Description of Related Art
The demand for an image forming apparatus capable of forming a color image (hereafter referred to as a color-image forming apparatus) for a color copying machine and a color printer has increased in recent years among image forming apparatuses for respectively forming an image in accordance with an electronic photographing system. As this type of the color-image forming apparatus, an apparatus is expected which can achieve such six items as (1) low running cost, (2) small space, (3) low power, (4) high image quality, (5) high speed, and (6) improvement of operability.
Therefore, to provide a high-quality color image at a high speed while simplifying operability, a system is used which forms an image by arranging four process cartridges for four colors such as yellow, magenta, cyan, and black respectively provided with a photosensitive member drum in parallel.
Moreover, to improve operability, a process cartridge system has been used so far which integrates a photosensitive member drum, charging device, and developing device into a cartridge so that the cartridge is detachably mounted to the body of an image forming apparatus. By using the above cartridge system, operability is further improved and a user can easily maintain a photosensitive member drum and process means (charging device and developing device) working on the photosensitive member drum.
Moreover, similarly to the above, in the case of an image forming apparatus of temporarily transferring toner images to an intermediate transferring body and then simultaneously transferring toner images of a plurality of colors to a transferring material, the intermediate transferring body is also constituted as a unit so that the intermediate-transferring-body unit is removable from the body of the image forming apparatus and operability and maintainability are improved.
In the case of the above conventional image forming apparatus, however, because process cartridges of four colors and the intermediate-transferring-body unit are made removable from the body of the image forming apparatus by also considering operability, the configuration of the body of the image forming apparatus becomes complex. Therefore, there is deterioration of the positional accuracies of the process cartridges and intermediate-transferring-body unit.
Particularly, a stable accuracy free from fluctuation is requested for the positional accuracy between the photosensitive member drum of the process cartridges of four colors and the intermediate transferring body (intermediate transferring belt) of the intermediate-transferring-body unit in order to realize a high accuracy and a high-image quality. However, because the photosensitive member drum and the intermediate transferring body are constituted so as to be removable from the body of the image forming apparatus, the number of components set between the photosensitive member drum and the intermediate transferring body increases and thereby, dimensional errors of the set components are accumulated and the positional accuracy between both tends to be deteriorated.
On the other hand, at the time of removing a process cartridge while a photosensitive drum contacts an intermediate transferring body from the body of an image forming apparatus, it is necessary to once separate an intermediate transferring body 309 from a photosensitive member drum as shown in FIG. 13. Therefore, an image forming apparatus is provided with a mechanism for contacting with or separating from an intermediate-transferring-body unit.
In this case, to simultaneously realize the contacting/separating function of the contacting/separating mechanism of the intermediate-transferring-body unit 309 and the removing configuration from the body of the image forming apparatus, it is necessary to position the intermediate-transferring-body unit 309 to the body of the image forming apparatus at a portion other than the center of a driving coupling 324 set to a body driving unit for driving the intermediate-transferring-body unit 309.
However, when a positional error occurs between the center of the driving coupling 324 of the body driving unit and the center of a coupling 327 of the intermediate-transferring-body unit 309, if the intermediate-transferring-body unit 309 is driving-connected to the body driving unit while contacting with the photosensitive member drum, connection may not be realized, irregular rotation of a driving roller may occur, or color shift may occur due to irregular conveying of an intermediate transferring body. Moreover, because a relative position with a photosensitive member drum is shifted, the transferability of a toner image may be deteriorated.
Furthermore, at the time of setting a positioning member 331 for the body of an image forming apparatus of the intermediate-transferring-body unit 309 to a frame 301 of the image forming apparatus as shown in FIG. 13, a high stiffness is required for the frame 301. However, because a large opening is formed on the frame 301 in order to set or remove process cartridges of four colors, intermediate-transferring-body unit 309, and other replaceable units to or from the frame 301, the stiffness of the frame 301 on which the large opening is formed is greatly deteriorated.
Therefore, the positional accuracy is deteriorated due to not only deterioration of the dimensional accuracies of the above component but also deformation of the frame 301 because of insufficient stiffness. Moreover, at the time of minimizing the plate thickness of the frame 301 in order to decrease the body of the image forming apparatus in weight and cost, the flatness of the frame is deteriorated and thereby, the positional accuracy of the intermediate-transferring-body unit 309 is more remarkably deteriorated.